I think 40K is perhapse the easier way into the table top gaming.
firstly its a lot cheaper which means that kids can soon collect a reasonable army and enjoy playing without having to fork out a small fortune (which most don't have).
Secondly its also a time consideration as well. You can set up and play a game of 40K in under an hour. most people don't have more than a couple hours to go and play a game. When it can take over 30 minutes to set up a fantasy game thats eating into precious gaming time.
Just rolling for terrain takes a long time and if your playing even a small fantasy game say 1500pts your deployment can take 20 minutes ( when you have to take out 100+ models and put them on the brd).
I like playing both games. I like my Eldar in 40k and I really enjoy the fluff and books so thats fun for me. but I love my daemons in 40k and in fantasy. tactically both can be a real challenge I'm newer to fantasy but so far i don't find it any harder. just more time consuming to do everything.